Early in the IUE mission it was discovered that the camera image
format shifts frequently in both the spectral and spatial directions with time
and camera temperature. Camera temperature was measured roughly by thermistors
located in any of several areas of the camera head amplifier (not an
ideal location to measure the temperature of the spectrograph bench itself),
and one of these was selected to provide a consistent refererence. This
provided a ``THDA" index which could be used to calibrate wavelength
zero-point shifts and apply them to the processing of a particular
observation in IUESIPS and NEWSIPS.
Motion of the spectral format on the detector was generally caused by
telescope flexures and, under routine operating conditions, secondarily
by changes in electro-optical properties of the camera. Thus, fluctuations
in satellite temperature (which was primarily a function of
the telescope's orientation angle with respect to the Sun) caused the
telescope to expand or contract and the focus to change.
The focus was usually controlled indirectly by turning on or off
a heater located behind the primary mirror. An alternate control
procedure consisted of toggling a heater mounted on the spectrograph deck.
Although the camera temperature and detector image shifts were correlated,
a hysteresis arising from the instrument's thermal history tended to
weaken the efficacy of controlling the focus by commands to the spacecraft.
This control was weakened further by other environmental factors, such as
the Earth's eccentric orbit around the Sun and earthglow. Consequently, the
IUE Operations team developed a conservative strategy of maintaining
the THDA temperature within certain limits. During the course of the IUE
mission statistical correlations were derived from shifts of WAVECAL
spectra as a function of the THDA and focus parameters.
These shifts were parameterized
by means of a polynomial fit, so NEWSIPS could remove them to first
order in its assignment of wavelengths.

As part of our cross-correlation analysis, we searched for
a correlation between zero-point shifts and THDA and focus
values at the time of an observation. Generally,
we were unable to find a convincing correlation.
Except for an accident of an unrelated contemporaneous research program,
this search might have stopped with this null result. However, while
investigating spectral variations in B stars for another research program,
we used the cross-correlation tools from the present study to shift spectra
to the same wavelengths before manipulating the data further. In doing so,
we noticed a time-dependent cycle of about a day
(see Figure 6)
in the results for a time series of 22 observations conducted in 1995
on the
B3Ve star Eri. In searching through the IUE archival database
further, we found an extraordinary set of 181 continuous observations on the
B0.5V star Per in 1996 obtained by Dr. D. Gies.
Figure 7 shows that the same one-day
cycle is present in the data for this star.
A string of observations of HD 93521 at 1994.2
(not shown) exhibits a similar 1-day period and few
km s-1 semiamplitude over three days.

Figure 6 Cross-correlation shifts of velocity, telescope focus, and THDA
temperature for an intensive time series of SWP high-dispersion
observations of Eridani. Dotted and dashed lines (shown for
reference only) represent the x- and y-components of the satellite's orbital
motion (``x" is directed toward the Sun; ``y" its perpendicular in the
Earth's equatorial plane).
THDA is in degrees Centigrade; focus is in instrumental units.

Figure 7 Cross-correlation shifts of velocity, telescope focus, and THDA
temperature for an intensive time series of SWP high-dispersion
observations of Persei. For reference only: a cross-correlation
between the apparent velocities and the x-component of the (diurnal) orbital
velocity shows that the velocities show a mean lag of 0.15 cycle behind the
x-component.

Because the cycle lengths in these shifts are all close to one day,
we investigated first whether the satellite's orbital motion might have
somehow been neglected in determining wavelengths for an individual
observation. However, we were able to discount that possibility.

A more successful attempt to explain the apparent 1-day velocity cycles of
Eri, Per, and HD 93521 was to investigate the effects
of varying the instrument temperature by adjusting the telescope focus.
Cycling the camera-deck heater produced correlated responses of the
THDA and telescope-focus values, especially when the
ambient spacecraft temperature was lower than nominal, as for the 1996
observations of Per. During this particular monitoring series,
the good correlation between temperature and focus indicates that the focus
was controlled by the deck heater. In the time series on Eri the
temperature was not quite so low. Then an adequate means of controlling
the focus was to cycle the more distant telescope mirror heater.
Since the telescope heater was isolated from the spectrograph, the locally
measured THDA value did not correlate well with the focus changes
of the telescope (see lower dashed line in figure). However, in either
case the principle was the same, that heating could be applied to prevent
the focus from drifting to large negative values. The important point is
that this application caused an overcorrection of the focus.
The focus values would then swing (relatively) positive until a low ambient
temperature again reversed the change of the focus, causing a new
thermal-focus cycle to ensue. All told, the changes in focus from either
thermal-control technique caused the velocity to become first too negative
and then too positive by a few km s-1. Although we have found the
velocity-focus correlation only in SWP camera datasets so far, it is
probably present in long-wavelength data as well.

Figure 8 exhibits the correlation explicitly between
telescope focus value and zero-point for the
observing campaign on Per. A similar plot can be constructed
for THDA instead of focus, but the Eri data imply that the
correlation of velocity with THDA for the Per data is a result
of temperature excursions changing the focus, and not a shift caused
by the temperature variation within the camera.
Note especially that Fig. 8
shows that the correlation arises only in the limited focus range of -2.0 to
-3.7 (instrumental units). As noted above, we also searched our results for
a dependence on focus and THDA values in our study of time-dependent
correlations, but we found none. We suspect the reason is that IUE
observations of most objects were obtained at different epochs
and with different target-centering practices.
This would tend to conceal any correlation over a small critical subrange
of focus values in our searches for trends in the stars of
Table 1.
However, we expect the pattern shown in
Fig. 8 is present in
all high-dispersion data to some extent. If so, it is a a secondary source
of radial velocity error.